riscv64: Implement fcmp in ISLE (#5512)

Rework the compilation of fcmp in the riscv64 backend to be in ISLE, removing the need for the dedicated Fcmp instruction. This change is motivated by #5500, which showed that the riscv64 backend was generating branch instructions in the middle of a basic block.

We can't remove lower_br_fcmp quite yet as it's used in a few places in the emit module, but it's now no longer reachable from the ISLE lowerings.

Fixes #5500

cranelift/filetests/filetests/isa/riscv64/fcmp.clif

@@ -0,0 +1,29 @@
+test compile precise-output
+target riscv64
+
+;; See #5500 for more details about this test case.
+function %f0() {
+block0:
+    v0 = f64const 0.0
+    v1 = fcmp ult v0, v0
+    brz v1, block1
+    jump block1
+
+block1:
+    return
+}
+
+; block0:
+;   li t1,0
+;   fmv.d.x ft1,t1
+;   li a2,0
+;   fmv.d.x ft5,a2
+;   fle.d a5,ft5,ft1
+;   bne a5,zero,taken(label1),not_taken(label2)
+; block1:
+;   j label3
+; block2:
+;   j label3
+; block3:
+;   ret
+